1. Field of the Invention
The present invention generally relates to a hydraulic brake system and, more particularly, to an automobile hydraulic brake system which gene rates a brake force by introducing fluid pressure generated by a high pressure source into a wheel cylinder.
2. Description of the Related Art
A conventional hydraulic brake system is described in Japanese Laid-Open Patent Application No.6-344894 which generates a desired brake force by introducing a hydraulic pressure generated by a hydraulic pump into a wheel cylinder when a brake pedal is operated. The hydraulic pressure introduced into the wheel cylinder is responsive to a force applied to the brake pedal.
In a hydraulic brake system of this type, since the hydraulic pressure introduced into the wheel cylinder is generated by the hydraulic pump, a large brake force can be obtained by applying only a small force to the brake pedal. Accordingly, a brake system having such a hydraulic brake provides a good brake feeling.
The hydraulic brake system described in the above-mentioned patent application has a spool valve which selectively connects one of the hydraulic pump and the reservoir tank to the wheel cylinder. One end of the spool valve is provided with a first pressure receiving surface onto which a master cylinder pressure generated by the master cylinder is applied. Thus, a force equal to a multiplication of the master cylinder pressure and the pressure reviving area of the first pressure receiving surface is applied to the spool valve. This force is hereinafter referred to as a spool valve driving force. The other end of the spool valve is provided with a second pressure receiving surface onto which a wheel cylinder pressure generated by the wheel cylinder is applied. Thus, a force equal to a multiplication of the wheel cylinder pressure and the pressure receiving area of the second pressure receiving surface is applied to the spool valve. This force is hereinafter referred to as a feedback driving force. When the spool valve driving force is greater than the feedback driving force, the spool valve is moved in a direction where the hydraulic pump is connected to the wheel cylinder. Or the other hand, when the spool valve driving force is less than the feedback driving force, the spool valve is moved in a direction where the reservoir tank is connected to the wheel cylinder.
According to the above-mentioned structure, when the spool valve driving force is greater than the feedback driving force, the wheel cylinder is connected to the hydraulic pump until the feedback driving force becomes equal to the spool valve driving force. On the other hand, when the feedback driving force is greater than the spool valve driving force, the wheel cylinder is connected to the reservoir tank until the feedback driving force becomes equal to the spool valve driving force. In this case, the feedback driving force accurately balances with the spool valve driving force. Thus, according to the above-mentioned hydraulic brake system, a precisely controlled pressure, which is responsive to a force applied by an operator and supplied to the wheel cylinder, can be generated by using the hydraulic pump as a high pressure source.
However, since one of the hydraulic cylinder and the reservoir tank is selectively connected to the wheel cylinder, the wheel cylinder pressure is continuously increased while the connection between the hydraulic pump and the wheel cylinder is selected. Thus, it is required to interrupt the connection between the wheel cylinder and the hydraulic pump, when the wheel cylinder pressure reaches a predetermined pressure relative to the master cylinder pressure, by applying to the spool valve the feedback driving force which is against the spool valve driving force generated by the master cylinder pressure. That is, a mechanism is needed to apply the wheel cylinder pressure to the spool valve as the feedback driving force which is against the spool valve driving force. In this respect, the above-mentioned conventional hydraulic brake system has room for improvement in that a complex mechanism is needed to control the wheel cylinder pressure.